If the private key is not passphrase protected just use the OpenSSH keygen import:

ssh-keygen -i -f my_tectia_privatekey

This will print the private key in OpenSSH format to stdout.

If the Tectia private key is passphrase protected you'll need to remove the passphrase first using Tectia keygen (just press enter when prompted for New passphrase):

$ ssh-keygen-g3 -e my_tectia_privatekey
Passphrase needed for key "My Tectia Private Key".
Passphrase :
Do you want to edit key "" (yes or no)? yes
Your key comment is "My Tectia Private Key". Do you want to edit it (yes or no)? no
Do you want to edit passphrase (yes or no)? yes
New passphrase :
Again :
Do you want to continue editing key "" (yes or no)? no
Do you want to save key "" to file my_tectia_privatekey (yes or no)? yes

Then follow the same step as above for unencrypted private keys.

ssh-keygen -i -f my_tectia_privatekey > my_openssh_privatekey

Finally, encrypt the key again with a passphrase using OpenSSH keygen: